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.Dd July 17, 2025
.Dt CORE 5
.Os
.Sh NAME
.Nm core
.Nd memory image file format
.Sh SYNOPSIS
.In sys/param.h
.Sh DESCRIPTION
A small number of signals which cause abnormal termination of a process
also cause a record of the process's in-core state to be written
to disk for later examination by one of the available debuggers.
(See
.Xr sigaction 2 . )
This memory image is written to a file named by default
.Nm programname.core
in the working directory;
provided the terminated process had write permission in the directory,
and provided the abnormality did not cause
a system crash.
(In this event, the decision to save the core file is arbitrary, see
.Xr savecore 8 . )
.Pp
The name of the file is controlled via the
.Xr sysctl 8
variable
.Va kern.corefile .
The contents of this variable describes a filename to store
the core image to.
This filename can be absolute, or relative (which
will resolve to the current working directory of the program
generating it).
.Pp
The following format specifiers may be used in the
.Va kern.corefile
sysctl to insert additional information into the resulting core
filename:
.Bl -tag -width "1234567890" -compact -offset "12345"
.It Em \&%H
Machine hostname.
.It Em \&%I
An index starting at zero until the sysctl
.Em debug.ncores
is reached.
This can be useful for limiting the number of corefiles
generated by a particular process.
.It Em \&%N
process name.
.It Em \&%P
processes PID.
.It Em \&%S
signal during core.
.It Em \&%U
process UID.
.El
.Pp
The name defaults to
.Em \&%N.core ,
yielding the traditional
.Fx
behaviour.
.Pp
The maximum size of a core file is limited by the
.Dv RLIMIT_CORE
.Xr setrlimit 2
limit.
Files which would be larger than the limit are not created.
.Pp
With a large limit, a process that had mapped a very large,
and perhaps sparsely populated, virtual memory region, could take
a very long time to create core dumps.
The system ignores all signals sent to a process writing a core file, except
.Dv SIGKILL
which terminates the writing and causes immediate exit of the process.
The behavior of
.Dv SIGKILL
can be disabled by setting tunable
.Xr sysctl 8
variable
.Va kern.core_dump_can_intr
to zero.
.Pp
By default, a process that changes user or group credentials whether
real or effective will not create a corefile.
This behaviour can be
changed to generate a core dump by setting the
.Xr sysctl 8
variable
.Va kern.sugid_coredump
to 1.
.Pp
Corefiles can be compressed by the kernel if one of the following items
are included in the kernel configuration file:
.Bl -tag -width "1234567890" -compact -offset "12345"
.It options
GZIO
.It options
ZSTDIO
.El
.Pp
The following sysctl control core file compression:
.Bl -tag -width "kern.compress_user_cores_level" -compact -offset "12345"
.It Em kern.compress_user_cores
Enable compression of user cores.
A value of 1 configures
.Xr gzip 1
compression,
and a value of 2 configures
.Xr zstd 1
compression.
Compressed core files will have a suffix of
.Ql .gz
or
.Ql .zst
appended to their filenames depending on the selected format.
.It Em kern.compress_user_cores_level
Compression level.
Defaults to 6.
.El
.Sh NOTES
Corefiles are written with open file descriptor information as an ELF note.
By default, file paths are packed to only use as much space as needed.
However, file paths can change at any time, including during core dump,
and this can result in truncated file descriptor data.
.Pp
All file descriptor information can be preserved by disabling packing.
This potentially wastes up to PATH_MAX bytes per open fd.
Packing is disabled with
.Dl sysctl kern.coredump_pack_fileinfo=0 .
.Pp
Similarly, corefiles are written with vmmap information as an ELF note, which
contains file paths.
By default, they are packed to only use as much space as
needed.
By the same mechanism as for the open files note, these paths can also
change at any time and result in a truncated note.
.Pp
All vmmap information can be preserved by disabling packing.
Like the file information, this potentially wastes up to PATH_MAX bytes per
mapped object.
Packing is disabled with
.Dl sysctl kern.coredump_pack_vmmapinfo=0 .
.Sh EXAMPLES
In order to store all core images in per-user private areas under
.Pa /var/coredumps
(assuming the appropriate subdirectories exist and are writable by users),
the following
.Xr sysctl 8
command can be used:
.Pp
.Dl sysctl kern.corefile=/var/coredumps/\&%U/\&%N.core
.Sh SEE ALSO
.Xr gdb 1 Pq Pa ports/devel/gdb ,
.Xr gzip 1 ,
.Xr kgdb 1 Pq Pa ports/devel/gdb ,
.Xr setrlimit 2 ,
.Xr sigaction 2 ,
.Xr sysctl 8
.Sh HISTORY
A
.Nm
file format appeared in
.At v1 .
